Almkvist's unimodality conjecture for two-variable Hilbert functions

Let m,nm,n be positive integers, let R=Q[x1,,xn]R={\mathbb Q}[x_1,\ldots,x_n] with its standard grading, and write ei=ei(x1,,xn)e_i=e_i(x_1,\ldots,x_n) for the elementary symmetric functions and ei(m)=ei(x1m,,xnm)e_i(m)=e_i(x_1^m,\ldots,x_n^m). Define

A(m,n)=Q[e1,,en](e1(m),,en(m))A(m,n)=\frac{{\mathbb Q}[e_1,\ldots,e_n]}{(e_1(m),\ldots,e_n(m))}

and let H(m,n)=(H(m,n)k)k=0dH(m,n)=(H(m,n)_k)_{k=0}^d be the coefficient sequence of its Hilbert polynomial.

Almkvist's conjecture. For each mm, the Hilbert function H(m,n)H(m,n) is unimodal for all n11n\geq 11. Moreover, if mm is even, then H(m,n)H(m,n) is unimodal for all nn.

These Hilbert functions also enumerate a two-parameter family of integer partitions and are related to the unimodality of graded Artinian complete intersections. The conjecture is attributed to G. Almkvist and is presented here without a stated resolution.

Sources & referencesView supporting material

Primary source

Nancy Abdallah and Chris McDaniel, “Lattice Paths, Lefschetz Properties, and Almkvist's Conjecture in Two Variables”, arXiv:2404.05098 (2024).

Additional references

4 papers in this index state this conjecture (2018–2024). The statement above is taken from the most recent of them; the others are arXiv:2311.03081, arXiv:2304.01032, arXiv:1807.05869.

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